1. Field of the Invention
This invention relates generally to electronic control of cooling fans, and more particularly to a system and method for reducing undesirable noise resulting from driving a fan with pulse width modulation.
2. Description of the Background Art
It is common practice to regulate the speed of fans used to cool electronic components such as computers. Regulating the speed of fans increases fan life, decreases noise caused by unnecessarily high airflow, and conserves electrical energy.
One common method of controlling fan speed is to drive the fan using low frequency pulse width modulation (PWM). According to PWM, electrical power is supplied to the fan in a series of pulses. The fan's speed is controlled by controlling the width (duration) of the pulses. The percentage of time that a pulse is being applied to the fan is referred to as the “duty cycle.” When the pulses are wide (high duty cycle), the fan speed is correspondingly high. When the duty cycle decreases (relatively narrower pulses), the fan speed correspondingly decreases.
Although PWM has proven to be one of the most efficient methods of controlling fan speed, objectionable noise, commonly referred to as “growling”, is generated when the duty cycle is reduced below approximately 30 percent. Typically, the noise occurs at a frequency equal to the PWM, and is especially noticeable when using high operating current fans, and low operating speeds. Accordingly, fan control circuits commonly require that the fan be driven with at least 30% PWM duty cycle, which may be faster than necessary or desirable for quiet operation.
What is needed, therefore, is a system and method for using PWM to drive a fan at low duty cycles, without causing growling noise. What is also needed is a system and method for using PWM to drive a fan at speeds lower than speeds corresponding to a 30 percent duty cycle.